1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device, which includes a process of annealing a semiconductor film using a laser beam (hereinafter referred to as laser annealing). Semiconductor devices herein include electro-optical devices such as liquid crystal display devices and light emitting devices, and electronic equipment that contains the electro-optical devices as components.
2. Description of the Related Art
In recent years, a technique of crystallizing a semiconductor film formed over an insulating substrate made of glass or the like, and improving the crystallinity of the film has been extensively studied. In order to accomplish the objectives described above, the film is heated and/or laser annealed. For forming the semiconductor film, it is a common practice to use silicon. In this specification, laser crystallization refers to a method of crystallizing a semiconductor film by means of the laser beam to obtain a crystalline semiconductor film. Note that, in this specification, the crystalline semiconductor film refers to the semiconductor film in which crystallized areas exist.
The crystalline semiconductor film formed by the crystallization method as described above possesses high mobility. For this reason, the crystalline semiconductor film is increasingly employed in various devices such as monolithic type liquid crystal electro-optical devices. In these electro-optical devices, thin film transistors (TFTs) are formed by means of this crystalline semiconductor film, and TFTs for driving pixels and TFTs for driver circuits are formed on a single glass substrate.
As described above, the crystalline semiconductor film has extremely better properties than an amorphous semiconductor film. For this reason, the above-mentioned study has been conducted. For crystallizing the amorphous semiconductor film by heating, for example, the heating temperature of 600° C. or higher and the heating time of 10 hours or longer, preferably 20 hours or longer were required. Among the substrates that can withstand the crystallization conditions is a quartz substrate, for example. The quartz substrate, however, is very expensive, and it was extremely difficult to process the quartz substrate into a large-area substrate in particular. The large-area substrate is especially essential for increasing production efficiency. Recently, there is a remarkable trend toward larger-area substrates so as to improve the production efficiency. Therefore, on production lines of a factory to be newly constructed, a substrate size of 600=720 mm is becoming standard.
Among glass substrates that have a comparatively high melting point is a 1737 glass substrate. A warping point of the 1737 glass substrate is 667° C., a cooling point or a temperature from which a change in shape of the 1737 glass substrate becomes manifest is 721° C., and a melting point of the 1737 glass substrate is 975° C. When the amorphous semiconductor film was formed on this glass substrate and then placed in the atmosphere at 600° C. for 20 hours, a contraction in the substrate could be recognized. However, any deformation that would affect the processes of manufacturing a semiconductor device was not seen in the substrate. The 20-hour heating time, however, was too long in consideration of volume production.
In order to solve the problems as described above, a new crystallization method was devised. Details of this method are described in Japanese Patent Application Laid-open Hei 7-183540. Now, this method will be briefly described. First, a very small amount of nickel, palladium, lead or the like is introduced into an amorphous semiconductor film for doping. In order to perform the doping process, a plasma CVD method, a vapor deposition method, an ion implantation method, a sputtering method, a solution applying method or the like should be used. After the doping process, when the amorphous semiconductor film is placed in a nitrogen atmosphere at 550° C. for four hours, for example, a crystalline semiconductor film with satisfactory characteristics can be obtained. Incidentally, the heating temperature and time most suitable for crystallization depends on the doping amount of the element and the states of the amorphous semiconductor film.
The above was a description about the method of crystallizing the amorphous semiconductor film by heating. On contrast therewith, since crystallization by laser annealing can impart high energy to the amorphous semiconductor film alone without excessively increasing the temperature of the substrate, it can also be employed for a plastic substrate or the like as well as a glass substrate having a low warping point.
A high-power pulse laser beam such as an excimer laser beam is employed for laser annealing. The laser beam is processed by an optical system so as to form the beam shape of a square spot several by several centimeters square (rectangular shape) or a line of ten centimeters or longer (linear shape) on an irradiation plane. Then, scanning by the laser beam is performed, or the position of the laser beam irradiation is moved relative to the irradiation plane. This method provides increased productivity and is more excellent from a commercial point of view. For this reason, this method is preferably used.
When the linear laser beam in particular is employed, laser beam irradiation onto the entire irradiation plane can be performed just by scanning in a direction perpendicular to the longitudinal direction of the linear laser beam. On contrast therewith, when the laser beam that forms the spot shape on the irradiation plane is employed, back-and-forth and left-and-right scanning is required. For this reason, the linear laser beam provides higher productivity. Since the direction perpendicular to the longitudinal direction of the laser beam is the most efficient scanning direction, this direction is employed for scanning. Due to the higher productivity, use of the linear laser beam obtained by processing the pulse oscillation excimer laser by an appropriate optical system in the laser annealing method is becoming mainstream in the technology of manufacturing a liquid crystal display device using TFTs.
For forming the crystalline semiconductor film, there is also provided a method of crystallizing the amorphous semiconductor film by heating and then further crystallizing the resultant film by laser annealing. With this method, the characteristics of the semiconductor film can be improved more than in the case where crystallization is performed either by heating or laser annealing. In this method, in order to obtain the improved characteristics, it is necessary to optimize both heating conditions and laser annealing conditions. Manufacturing of thin film transistors (TFTs) using the crystalline semiconductor film obtained by the above-mentioned method greatly improves the electrical characteristics of the TFTs.
However, since the crystallization method by means of laser beam irradiation can impart high energy to the semiconductor film without excessively increasing the temperature of the substrate, an abrupt temperature gradient is produced between the substrate and the semiconductor film. Consequently, the semiconductor film contracts under a tensile stress and then becomes warped.
In addition, the semiconductor film becomes more compact by crystallization. This phenomenon can be confirmed from a decrease in thickness of the film. As described above, the semiconductor film contracts by crystallization, thereby becoming a factor for bringing about warps in the film.
These warps can also be confirmed by conducting Raman scattering spectroscopy, and then detecting a shift in a peak in the Raman spectrum after the laser beam irradiation.
Though these contractions are not significant at the stage where the substrate is transported, they have adverse effects on the characteristics of the semiconductor device of an insulating gate type. To take an example, a potential barrier or trapping levels resulting from the warp of the semiconductor film are induced, which leads to a higher interface level between an active layer and a gate insulating film. Further, when the semiconductor film is warped, uniform application of an electric field cannot be performed, which leads to a malfunction of the semiconductor device. In addition, the warp in the surface of the semiconductor film impairs the flatness of the gate insulating film deposited by the sputtering method or the CVD method, and causes faulty insulation or the like, thereby reducing reliability. A surface scattering effect is pointed out as one of the factors for determining the TFT's field effect mobility. The flatness of the interface between the active layer and the gate insulating film of the TFT greatly affects the field effect mobility. The flatter the interface is, the lesser the TFT is not affected by scattering, so that high field effect mobility can be obtained. In this way, the warp in the semiconductor film affects all the characteristics of the TFT, thereby changing even production yields.